The present invention relates to industrial chemical processes in which a processing chemical initially in the form of a liquid or solid is converted to the gas or vapor state, in which state it is conveyed to a processing station.
In many industrial chemical processes, a processing substance is stored in a container in a liquid or solid state and this supply of processing substance is gradually exhausted as the process in which it is utilized proceeds. When the supply of processing substance is completely exhausted, it is frequently necessary to halt the process in order to replace or replenish the container. When the process is allowed to continue until the supply of processing substance is completely exhausted, there can be a period of time during which unacceptable results are produced.
In the case of many processing substances, known techniques for monitoring the fill level of the container are not well suited because level sensors introduced into the container may react in an unacceptable manner with the substance. This could contaminate the substance and/or deteriorate the measuring device. In addition, mounting of the measuring device in the container may create substantial problems in connection with sealing of the container at the point where the measuring device or its connecting lines pass into the container.
One significant example of the problems described above is found in semiconductor fabrication procedures in which dimethyl aluminum hydride (DMAH) is delivered in the form of a vapor to wafers in order to form aluminum films thereon. DMAH is a highly flammable, explosive and toxic substance. Therefore, its container should be securely and permanently sealed during use. Vapors of this substance are produced by introducing a gas via a conduit which extends below the surface of the substance and which bubbles through the substance to produce the vapor. Since this vapor will fill the container, any devices installed in the container may acquire an aluminum coating. Therefore, it is believed that a satisfactory technique for monitoring the quantity of substance remaining in the container has not heretofore been devised.
If an attempt is made to utilize all of the substance in the container, and no means are available for monitoring the quantity of substance remaining in the container, complete exhaustion of the substance will only become apparent from the fact that the wafers being fabricated are defective. Given the realities of semiconductor fabrication procedures, this fact may not become apparent until a large number of wafers have proceeded through the fabrication process and have become defective resulting in a considerable economic loss which may be as high as tens of thousands of dollars.
In view of this difficulty, one practice in the art has been to make a prediction as to when a safe proportion of the substance in a container has been used and, at that time, to replace that container with a fresh container and return the used container to a supplier for refilling. The result is that a substantial amount of the substance, which is itself expensive, is not used.
Thus, whether a semiconductor fabrication process is performed in a manner which uses all of the substance in a container or in a manner which uses only a selected portion of that substance, the result can be a considerable economic loss.